Decontaminating Light Water Reactor (LWR) plant sub-systems have become relatively common in the United States and are widely recognized as a useful contributor to the reduction of radiation exposure at these plants. Sub-system decontamination involves exposing a part of the reactor circuit to chemical solutions to dissolve the radioactive deposits that have accumulated on the surfaces of the process equipment, including piping. The spent decontamination solutions are then treated by ion exchange to retain all the chemical and radioactive burden of the decontamination solution on the resin, while clean water is returned to the system. An example of such a process is the Low Oxidation-state Metal Ion reagents (LOMI) process, described in U.S. Pat. No. 4,705,573, hereby incorporated by reference.
The LOMI process removes deposits consisting essentially of the oxides of one or more transition metals from a surface. The process comprises the steps of contacting the surface, at a pH in the range of 2.0 to 7.0, with a reagent comprising a one-electron reducing agent. The reducing agent is the complex formed between a low oxidation state transition metal ion and a complexing agent, for destabilizing the metal oxides deposited, thereby increasing their rate of dissolution. The complexing agent is thermally stable at the operating pH and present in order to form the necessary complex reagent and also to increase the thermodynamic solubility of the metal ions released.
Decontamination processes of the type described above can also be applied to the whole of the reactor circuit (including the reactor core) in the absence of the fuel. This is sometimes called "Full System Decontamination." There are examples of this type of operation described in the literature such as the decontamination of the Indian Point 2 PWR in March 1995. See, Parry, J. O., Trovato, S. A., "National Demonstration of Full Reactor Coolant System (RCS) Chemical Decontamination--Post Decon. " EPRI Radiation Field Control & Chemical Decontamination Seminar, Tampa, Fla., Nov. 6-8, 1995. The only major differences between this type of operation and the sub-system decontamination described in the previous paragraph are the greater extent of the reactor systems exposed to the decontamination solution, and the use of the reactor coolant pumps to circulate the decontamination solution. This provides higher flow velocity and improved contact between the reactor surfaces and the decontamination solution. The main advantages of this type of operation compared with the traditional sub-system decontamination processes are the reduction of radiation dose rates in many different plant locations and the removal of radioactive material from the reactor core. This radioactive material would otherwise be available to re-contaminate the circuit, and thus the benefits of the decontamination continue to be gained in later maintenance outages after further periods of operation. The plant systems can be used to circulate the decontamination reagent, to control its temperature and possibly even to effect solution clean-up and therefore require less temporary equipment to carry out the decontamination. The main disadvantages of this type of full system decontamination are the large volume of radioactive waste ion exchange resin generated and the time required to unload the fuel from the reactor.
However, the fuel does not necessarily need to be unloaded from the reactor core prior to full system decontamination. The principal reasons for unloading the fuel before the decontamination process are concerns that the fuel might be damaged by the decontamination process and that the radiation from the fuel might degrade the decontamination reagent. Unfortunately, as stated previously, the removal of the fuel from the reactor takes significant time which, if it is on the "critical path" of the maintenance outage, can make the decontamination process noneconomical. If it were possible to leave the fuel in place, the decontamination could be done during the period immediately following reactor shut down, but before the reactor vessel head is removed. In this way, the time penalty can be avoided or at least minimized.
Referring to the "fuel-in" disadvantages above, concerns about the exposure of the fuel to the decontamination solution can usually be overcome, since the decontamination solutions are designed to be compatible with the fuel materials. There are many reports of the successful exposure of nuclear fuel elements to the decontamination reagent systems typically employed, and there are reports of typical decontamination processes becoming satisfactorily qualified for exposure to fuel elements. See Miller, P. E., "Fuel-In Full RCS Chemical Decontamination Qualification Program," EPRI Radiation Field Control & Chemical Decontamination Seminar, Tampa, Fla., Nov. 6-8, 1995. The extra radioactive material removed may cause some difficulties in waste management, but the purpose of decontamination is to remove radioactive material from the reactor coolant circuit, and the extra radioactive material removed by cleaning the fuel may in fact be an advantage. The final potential disadvantage of fuel-in decontamination, namely the exposure of the decontamination reagent to the radiation dose from the fuel, has been discussed in many reports, but has not previously been seen as a critical disadvantage. There are reports of successful full system decontaminations of reactors with fuel in place, in Canada (see Speranzini, R. A., Lister, D. H., "Canadian Experience with Full System Decontamination," 3rd EPRI Seminar on Chemical Decontamination of BWRs, Charlotte, N.C., 1988), in England (see Nash, G. J. C., "Decontamination of the SGHWR Prototype," Conference on Water Chemistry of Nuclear Reactor Systems. British Nuclear Energy Society, UK, 1977. Paper No: 45) and elsewhere.
It should further be noted that radiation can cause degradation of organic chemical reagents used for decontamination. In consideration of the behavior of the decontamination reagents in the presence of high radiation dose rates, the usual approach has been to apply the decontamination process in such a way that the radiation dose rates are not sufficient to cause significant decomposition. Alternatively, it may be accepted that decomposition occurs and the decontamination reagent must be replenished by fresh additions of chemicals during the application of the process.
In studies of the LOMI decontamination process, it was found that the behavior of the decontamination reagent could be controlled in the presence of radiation by converting oxidizing hydroxyl radicals produced by radiolysis into the carbon dioxide radical anion through reaction of the former with formic acid. Not only does this procedure protect the decontamination reagent against radiolytic damage, but the principal active ingredient of the reagent, vanadous picolinate, is formed from the spent reagent by reaction of the latter with the reducing radicals produced in this way. See Bradbury, D., Segal, M. G., Sellers, R. M., Swan, T. & Wood, C. J., "Development of LOMI Chemical Decontamination Technology," EPRI Report NP 3177, 1983.
What is needed is a process for minimizing the radioactive waste produced during the decontamination of nuclear reactors.